Conventional solar cells are unable to productively harness photons with energy below the bandgap of its active layers. As a result, large swaths of the solar spectrum are wasted, partly accounting for why the maximum theoretical power conversion efficiency (PCE) of conventional solar cells cannot exceed 32% under standard conditions. The intermediate band solar cell (IBSC) concept circumvents this limitation by introducing a narrow band of states within the bandgap of the absorber material that serves as a stepping-stone for the absorption of low energy photons that would have otherwise been lost. While such a device is able to utilize sub-bandgap photons, charges are still extracted via the outer band edges, so that the enhancement in photocurrent due to increased absorption is not achieved at the expense of photovoltage.
To date, demonstrations of IBSC devices are dominated by inorganic semiconductor approaches, in which the intermediate band states are typically achieved by extreme lattice mismatch or dense sheets of quantum dots. However, this approach has historically been frustrated by materials limitations and fabrication challenges. Moreover, charge recombination via the intermediate band has proven difficult to avoid. In 2008, the theoretical foundations were laid for an IBSC based on molecular materials and triplet-triplet annihilation upconversion (TTA-UC). It was proposed that the molecular triplet level of an organic material (the TTA host) serves as the intermediate band. The molecular IBSC is fundamentally different from its inorganic relative in that its intermediate band comprises long-lived (radiatively dark) triplet states rather than a partially-filled band of free carrier states. This difference constitutes a practical advantage for implementing the IBSC because it simplifies the requirement of preventing electrical recombination through the intermediate band since relaxation of triplet excitons to the singlet ground state is spin-forbidden. Several molecular IBSC devices have been constructed employing small molecule sensitizer and acceptor pairs in conjunction with liquid electrolyte. The requirement of liquid electrolyte presents several disadvantages including increased fabrication costs and limited device durability.